1. Field of the Invention
The present invention relates to a signal-reception circuit for receiving a signal and outputting the signal to a signal-processing unit, and also relates to a communication device including such a signal-reception circuit.
2. Description of the Related Art
FIG. 9 shows the essential components of a signal-reception circuit for receiving a signal, e.g., in a communication device. A signal-reception circuit 20 includes a triple band type antenna 21 which can receive signals in three different frequency bands. A reception signal S output from the triple band type antenna 21 includes mixed reception signals in the three frequency bands. In this example, a triplexer 22, which is a filter, is arranged to wave-separate the reception signal S into the reception signals in the frequency bands, respectively. The triplexer 22 separates the reception signal S transmitted from the antenna 21 into the reception signals in the frequency bands and supplies the reception signals to the signal-processing units (not shown), respectively.
Specifically, in the case in which the communication device having this signal-reception circuit 20 is a portable telephone having a GPS (Global Positioning System) function, the antenna 21 is configured so as to receive a PCS (Personal Communication System) signal, an AMPS (Advanced Mobile Phone Service) signal, and a GPS signal. The triplexer 22 separates the PCS signal, the AMPS signal, and the GPS signal of a reception signal S from each other, and supplies the PCS signal to a PCS signal-processing unit, the AMPS signal to an AMPS signal-processing unit, and the GPS signal to a GPS signal-processing unit, respectively.
Japanese Unexamined Patent Application Publication No. 11-150496 will be referred to as Patent Literature 1 and Japanese Unexamined Patent Application Publication No. 10-285093 will be referred to as Patent Literature 2.
In the configuration shown in FIG. 9, the triplexer 22 is provided as a wave-separating filter. The triplexer considerably increases the insertion loss for a signal in the middle frequency band of signals in the low, middle, and high frequency bands. Therefore, the level of the signal in the middle frequency band, output from the triplexer, is reduced. Thus, the configuration of FIG. 9 has a problem in that the reception sensitivity for a signal in the middle frequency band is deteriorated.
The configuration which can prevent the above-described problem is shown in FIG. 10 by way of example. According to this configuration, two antennas 24 and 25 are provided. The antenna 24 can receive, e.g., signals in two different frequency bands. The antenna 25 can receive signals in a frequency band, which is different from each of the reception frequency bands of the antenna 24.
A signal output from the antenna 24 is a mixed signal including signals in the two frequency bands. A diplexer 26 is arranged to separate the signals in the two frequency bands from each other. The diplexer 26 includes the combination of a band-pass filter for transmitting a signal in one of the reception frequency bands of the antenna 24 and a band-pass filter for transmitting a signal in the other reception frequency band of the antenna 24. Reference numeral 27 in FIG. 10 designates a band-pass filter for transmitting a signal in the reception frequency band of the antenna 25.
According to the configuration of FIG. 10, the diplexer 26 is used instead of the triplexer. This solves the problem in that the reception sensitivity for signals is deteriorated, due to the large insertion loss, which will be caused by the triplexer.
According to the above-described configuration, when the communication device is, e.g., a portable telephone, one (e.g., antenna 24) of the antennas 24 and 25 is arranged so as to project outward of a casing 28 for the communication device, and the other antenna (e, g., antenna 25) is disposed inside the casing 28. Accordingly, for operation of the portable telephone, a user grasps the casing 28, and in some cases, the antenna 25 is disturbed by the user's hand so that the reception condition of the antenna 25 is deteriorated. Thus, a problem occurs in that the reliability of the antenna for the reception sensitivity becomes low.